The present invention relates to a method of detecting focusing conditions used for optical machinery such as cameras, microscopes, endoscopes or the like, and more particularly to a focusing condition detecting method in which an optical image of a subject is projected onto a pair of photodetector arrays each including a plurality of photo-electro transducer elements arranged on both sides of a predetermined focal plane by means of an imaging lens, and output signals transduced by the photodetectors are arithmetically operated to derive values of a contrast evaluation function for the image projected onto the photodetector arrays, thereby detecting the focusing conditions for the imaging lens.
A method of detecting focusing conditions such as the method described in Japanese Patent Application Laid-Open No. 57,809/80 has been developed.
In the above mentioned method, use is made of a pair of photodetector arrays arranged on both sides of the focal plane of the imaging lens at a certain optical distance, signals derived from the photodetector arrays are converted into digital signals, and sums of difference signals having values from the maximum value down to Nth largest value therefrom are calculated from absolute values of difference between the output signals delivered from adjacent photodetectors. The focusing point is thereby detected as a position where the sum F-1a for the first photodetector array is made equal to the sum F-2a for the second photodetector array.
This method utilizes a pair of photodetector arrays to automatically control the focusing conditions for the imaging lens. If the focusing conditions are only detected, however, it is only sufficient to use one photodetector array so that the focusing condition detecting method in case of utilizing one photodetector array is explained hereinafter, in order to clarify the problem thereof in comparison with the present invention.
For convenient explanation, it is assumed that the evaluation function takes only the maximum value selected from the absolute values of differences between output signals of adjacent photodetectors.
FIG. 1 shows the relationship between the image space of subject image projected onto the photodetector array of the imaging lens and the light intensity distribution of the image by plotting photodetector arrays P1 to P10 on the abscissa and outputs of respective photodetectors on the ordinate. That is, FIG. 1A shows the state that optical reflection density of the subject is changed in step like. In this stepped change portion of reflection density the output signals of adjacent photodetectors P5 and P6 correspond to a maximum (Imax) and a minimum (Imin) of light intensity distribution, respectively.
In this case the evaluation function F-1a of the photodetector array represented by the maximum value among the absolute values of difference between outputs of adjacent photodetectors, exhibits a sharp peak near the position that the subject image is focused at a plane of the photodetector array as shown in FIG. 2 and the peak value thereof is large as shown by [F-1a(peak)=.vertline.Imax-Imin.vertline.].
On the other hand, when reflection density of the subject is graded or gently changed, the light intensity distribution of the image is obtained as shown in FIG. 1B. In this case the evaluation function F-1b of the photodetector array has a peak near the focusing position as shown in FIG. 2 and its peak value [F-1b(peak)=.vertline.I.sub.(P6) -I.sub.(P7) .vertline.] is very small as compared to the peak value F-1a.
As is seen from FIG. 2 the evaluation function has a relatively low level at the position more apart from the focusing point and this level fluctuates due to the difference in properties of respective photodetectors (P1 to P10) and the quantizing noise or the like caused at A/D conversion of the outputs from respective photodetectors, so that it is difficult to detect the focusing conditions for the graded subject. Therefore, when use is made of a pair of photodetector arrays and the position equal to the values of evaluation function obtained from the outputs of respective photodetectors is detected as a focused position, it is difficult or sometimes impossible to decide the focusing conditions when the level is less than a certain value of the evaluation function.
FIG. 2 shows various evaluation functions obtained by using a pair of photodetector arrays arranged on both sides of a predetermined focal plane at a certain distance along an optical axis of the imaging lens and by applying the above evaluating method. In FIG. 2 the evaluation functions for the stepped light intensity distribution of the image are shown by F-1a and F-2a, and the evaluation functions for the graded light intensity distribution of the image are shown by F-1b and F-2b, respectively. The position at which the values of evaluation functions obtained for the photodetector arrays are equal to each other is detected as an in focus position for the imaging lens.
In a common object, the boundary between two objects, or the boundary between hair and a face is considered to be the stepped subject, while the portion forming gradually deep shadow such as cheeks or a nose in a face is considered to be the graded subject.
In the above described conventional method only the maximum value of the absolute values of differences between output signals of adjacent photodetectors is taken as the value of the evaluation function. In order to obtain the value of the evaluation function of a subject image having stepped change portion of reflection density the pitch of photodetectors must be made smaller. Then, the difference between outputs of adjacent photodetectors also becomes smaller, so that the value of the evaluation function becomes smaller as shown in FIG. 2, which results in a difficulty of detecting the focusing conditions in the case of the graded subject image.